This Invention relates generally to a hydrometallurgical zinc production process by which zinc and other valuable metal elements such as lead, gold and silver, as well as the by-product elemental sulfur (which is hereunder referred to simply as xe2x80x9csulfurxe2x80x9d) are recovered from zinc concentrates which are sulfides containing not only zinc but also other valuable metal elements such as lead, gold and silver. In particular, the invention relates to the step of leaching zinc concentrates in the process.
Known prior art techniques for leaching zinc concentrates and other steps in the hydrometallurgical zinc production process are disclosed in JP No. 2,856,933 and JP 6-43619B. In the method of treating zinc concentrates according to JP No. 2,856,933, the leaching step is carried out in two stages. Prior to leaching, the zinc concentrates are roasted to form calcined zinc (calcines) which are then subjected to neutral leaching. In the next stage, using the spent electrolyte (return acid) from the electrolytic winning step, strong acid leach is effected to dissolve the unleached zinc concentrate and the slightly soluble zinc ferrite which results from the roasting step. The ion of trivalent iron (sometimes called xe2x80x9cferric ironxe2x80x9d) which is necessary to leach zinc is not fully supplied by the iron that occurs from the decomposition of zinc ferrite, so after the leaching step, the ion of divalent iron (sometimes called xe2x80x9cferrous ironxe2x80x9d) is recycled after oxidization. As the result of performing this leaching process for 6-10 hours at 90-95xc2x0 C., a zinc recovery of about 99% can be achieved. The residues from the leaching step are subjected to either a pyrometallurgical treatment in a blast furnace to recover any valuable metals present or flotation to concentrate the valuable metals for subsequent recovery.
In the method described in JP 6-43619B, the step of leaching zinc concentrates consists of at least two stages. After the zinc concentrate is pulverized into fine particles, the first stage of leaching is effected under superatmospheric condition by applying an oxygen pressure at a temperature of 125-160xc2x0 C. to give a final free sulfuric acid concentration of 20-60 g/L and a ferric ion concentration of 1-5 g/L. This results in incomplete leaching of zinc. The second stage of leaching is effected under atmospheric pressure using an excess amount of the spent electrolyte (return acid) generated in the electrolytic winning step, with oxygen being supplied to give a free sulfuric acid concentration of 60-160 g/L and a ferric ion concentration of 2-3 g/L. This yields a solution of zinc sulfate and a leach residue. Since the residue contains the remaining part of zinc, copper and iron, as well as the greater part of lead and noble metals, flotation is applied to recover these metal values as separate entities.
The prior art methods described above have the advantage that they can be incorporated into the existing circuit of roasting, leaching and electrolytic winning steps in the hydrometallurgical zinc producing process and that the need to reinforce the existing equipment is fairly small. In addition, the percent zinc recovery from the zinc concentrate is fairly high and it is also possible to recover valuable metal elements such as lead and precious metals. However, the prior art methods have their own drawbacks. To implement the method disclosed in JP No. 2,856,933, roasting equipment, sulfuric acid recovering equipment and even equipment for oxidizing the ion of divalent iron have to be added, leading to a higher construction cost; in addition, it takes an unduly long time to achieve complete leaching of zinc into solution. The method disclosed in JP 6-43619B has the following disadvantages: the pulverizing step is necessary; the leaching step involves a multiple of stages; the leaching temperature is so high that the operating cost is increased; the percent zinc leach is not high enough and, what is more, the sulfur which is a by-product forming as the zinc concentrate is leached is melted in the hot leach liquor and the reaction for leaching zinc from the zinc concentrate is inhibited, thereby prolonging the leach time while contributing to a further decrease in the percent zinc leach.
In order to solve these problems of the prior art, the present inventors conducted intensive studies and found that by grinding and leaching the zinc concentrate either simultaneously or separately, the ingredients that stayed on the surface of the concentrate to interfere with the leaching reaction can be effectively stripped or separated away to achieve a marked improvement in the rate at which zinc is leached.
When oxygen was fed into the piping to pressurize its interior while the leach liquor was being circulated by means of a pump, the iron ion that had been consumed to the divalent form during the reaction was regenerated to the trivalent form which could be put again into the leaching cycle, thereby successfully oxidizing the iron ion without using any large-scale pressure vessel.
The present inventors also attempted the following process: oxygen was supplied in a pressurized atmosphere to oxidize a liquor containing both free sulfuric acid and the divalent iron ion, thereby preparing an iron-containing, acidic solution into which the zinc concentrate was subsequently charged for leaching or, alternatively, the zinc concentrate was directly leached in the pressurized atmosphere, thereby causing the oxidization of the ferrous ion in the liquor and the leaching of the zinc concentrate under superatmospheric pressure to be accomplished in one step; thereafter, the zinc concentrate was leached as it was ground and then it was leached with the iron-containing, acidic solution or subjected to another cycle of oxidizing and leaching steps in the pressurized atmosphere; by means of this approach, both the leach rate and the percent leach were remarkably improved for not only zinc but also other valuable metals such as cadmium and copper in the zinc concentrate.
When a pressure vessel such as an autoclave having oxygen gas inlet ports, a drain port through which the iron-containing, acidic solution or the leached slurry was to be discharged, and any other necessary ports provided at specified sites of the vessel was used to implement the above-described leaching method under superatmospheric pressure with oxygen gas being supplied through the oxygen gas inlet ports, the rate of reaction for the oxidation of the ferrous ion was sufficiently increased that not only the rate of the reaction for the leaching of the zinc concentrate but also the percent leach could be improved to realize a substantial reduction in the scale and the cost of the leaching equipment.